Microalgae (broadly defined herein to include photosynthetic single celled eukaryotic algae and cyanobacteria) have a very high growth potential; more than ten times the productivity per unit area compared to terrestrial crops. Microalgae are potentially appropriate raw materials for producing low cost biofuels, animal feeds, and other products. Various impediments have prevented achieving this potential; the biological impediments of using undomesticated organisms are being overcome by genetic engineering of the microalgae (Gressel 2013). The impediment of the high cost of algal harvesting (dewatering) by high-speed (energy intensive) centrifugation has been overcome by a novel flocculation technology that is dependent on cultivating dense cultures of the microalgae (US2011/081706). The major remaining impediments are in cultivation; both in the expense of the structures used, and the high costs of running them. Open raceway ponds and their derivatives are inexpensive to construct, but must have a depth of at least 40 cm to allow adequate mixing and dissolution of bubbled (sparged) carbon dioxide. Even then, a considerable amount of carbon dioxide is lost to the atmosphere. The algae must be kept relatively dilute to allow light penetration, and even then all photons are typically absorbed in the upper 5-10 cm, and the material below respires photosynthate, decreasing yield from its potential. There is a high cost in sterilizing the large volumes of water used, in the compressors needed for bubbling, in the paddle wheels for mixing, and for the unused carbon dioxide lost. Cooling of open raceways is inexpensive in dry climates, i.e. is evaporative, requiring replacement with fresh water, even with marine algae to prevent over salinization. Open systems are easily contaminated by other unwanted species, including other algae, microbes, and algae-eating protozoan and metazoan herbivores, and thus various closed systems have been designed, but cooling is especially expensive for closed systems, because the deep water adsorbs infrared radiation that causes heating, which cannot be dissipated by evaporative cooling.
A generic diagram of such a closed bioreactor is shown in FIG. 1, and how it fits in a general cultivation system is outlined in FIG. 2 A, B, C. This design differs substantially from the many designs proposed for photobioreactors (see Table 1 below), as the algae herein are cultivated in a concentrated thin layer, which together with the use of near shear wave motion, renders sparging as unnecessary.
TABLE 1Designs of sun-lit photobioreactors with insufficient light penetrationto most cells in dense culture and inefficient carbon-dioxide mixingPhotobioreactorOptical pathconfigurationthickness (mm)Carbon dioxide supplyCommentsReferencecVerticalbAirlift typeConcentric tube Bubble-130 spargingContreras et al. (1998)column in airliftMerchuk et al. (2000)TubularTubular50-600Mixing bubblesPCT/US2009/056747Bags Submerged in waterNot statedTyvek tube spargersPCT/US2009/046782Helical tubular30Sparging in airliftHall et al. (2003)Parallel tubularOlaizola (2000)Tubes in manifold25-50 BubblingMixing airUS 2011/0104790with/without CO2Hanging bag100-200 Air flowMoheimani 2012PlatesParallel platesndgSpargingTiltableUS 2011/051507Parallel rigid plates 10-1000SpargingPCT/US2011/040366Parallel plates100-250 SpargingFloatingDE 102008/022 676Flexible parallel plates50-60 BubblingInternal heat exchangerWO 2005/006838Flexible Parallel platesJune 25JetsSqueeze mixingWO 2009155032Flat plate100 SpargingCheng-Wu et al. (2001)Flat platePCT/US2011/040366Variable flat plate floating62SpargingIn parallel clustersUS 2011/0281340Thin layer flat plate10SpargingExperimentalXue 2011Variable flat plate cluster100-20 in clusterUS 2010/0028976Flat plate15-25 BubblesZhang et al. (2002)On solidsHollow trabeculaendgSpargingPivotingUS 2011/0306121Various2-7 SpargingSupplementary lightingEP 1 995 304Fabric sheetsThin layer on fibersDiffusion from airWO 2011/138477Immobilized fibersThin layer on nonIn flow liquidU.S. Pat. No. 7,745,201woven fibersInclinedbInclined tubular100 SpargerWith mixing bafflesUS 2005/026053Inclined tubular38-125SpargerStatic mixer in airliftUgwu et al. (2003)Tubular -airlift300-1000SpargerPCT/US2005/025367Tubular 50-1000“Introduced”With cleaning vanesAppl. GB 2,330,589Open - thin layer5-18Sparging inDown hill flowU.S. Pat. No. 5,981,271recirculationCascade of flat20Sparging inHeliostat conc.US 2008/0293132platesrecirculationLightAirlift - flat plate65Bubbling2011/0159581Tilted flat plate19Spraying algalUse channelsUS 2011/0312062LED lit + solarsuspensionTilting flat plate manifold>100 Sparging andCreates travelling wavePCT/US2011/036527wave mixingthrough channelsUS 2011/0281339Tilting and ribbedVariable diaphragmFan with bubbling holesUS 2009/0203067HorizontalTroughs/PondsRaceway500 No mentionMixing by von KarmanUS 2008/086939vorticesAlgae inoculatedDeepBy bacteria andLagoon has co-culturedopen lagoonspargingalgae and bacteriaOpen V-trough600 at bottomSpargerUncoveredUS 2009/0215155Open & covered V-trough50-250Bubbling linesUS 2012/0064508Sequential open troughsNot statedNot statedA propagation conceptPCT/NL01/00273Solar film covered racewayDeepSpargedFilm adsorbs IRDE 102009015925Domed pondDeepSparged separatelydimensionlessUS2010/255569TubularExternal-loop tubular53 riserSparging in attachedHorizontal submerged/Acien Fernandez et30-160 horizontalairliftAirlift riseral. (2001)Double jacketed tubularNot givenSparged + MixingUse light concentratingU.S. Pat. No. 5,958,761vanesparabolic mirrorFloating tubular or sleeves20-200Sparging and mixingUS2009/0130706Immersed tubes300-1200pumpedInflated sidePCT/NL/2008/050650chambers forFlexible tubes or150SpargingCirculating algaeUS 2008/0311649sleevesRigid tubes10-60 Not consideredPVCUS 2010/0144023Rigid tubes50-300Sparge in separatePVCUS 2011/0104796structureSheets/sleevesFlexible sleeves on ground300 Flexible diffusersPaddle wheel mixingWO 2010/012028Floating flexible sleevesNot stated, but“Introduced”Buoyancy from airspaceWO 2009/087567not thin layerManifolded wide sleeves26-150“In degas vessel”Slightly tiltedUS 2010/0248333Flat sheet5-30DiffusionUS 2011/0217692Flat plate/rigid100-1300SpargingMixing by flow barriersUS 2009/0068727cover/closedFlat sheet130-350 SpargingIn viscous liquid SonicUS 2011/0092726waves to obtain ripplesfor better lightabsorptionFlat sleeve25-250“Fed”Water cooledUS 2011/0065157Flat sleeveNot statedSparge and membraneSunlight flickered byWO 2011086358lenses and flowFloating sleeves20-200Pressurized CO2Semipermeable underUS 2010/0216203lower sheetWO 2010/065862UnderwaterNot statedBubblingPseudo airlift beneathUS 2012/0107452growth chamberSleeves on soil200-300 spargerRoller mixing from topUS 2007/0048848Covered raceway floating200 Spraying algae torigid cover/U.S. Pat. No. 8,110,395or on groundheadspace &spargingCircular floating>>20 mmSparger in rodsMixing by rotating rodsUS 2012/0115210covered pondsrod diameter (ns)Floating tubes in matNot statedSparger in recycleFish eat biofilmAppl GB 2473865reservoirFloating sleeves200 Sparger or fountainMany possibleUS 2008/0009055mixers citedSubmersible50Sparger orMulti-compartmentedUS 2011/0124087floating sleevesdiffusionaBioreactors predominantly illuminated internally or externally by fluorescent, LED, fiber-optics etc. artificial light are excluded from this table.bOnly representative examples of super-structure requiring vertical and incline photobioreactors are given, because they are not the subject of this applicationcNon-patent citations are listed at the end of the application in the general references
Bioreactors predominantly illuminated internally or externally by fluorescent, LED, fiber-optics etc. artificial light are excluded from this table.
b Only representative examples of super-structure requiring vertical and incline photobioreactors are given, because they are not the subject of this application
c Non-patent citations are listed at the end of the application in the general references Water adsorbs infrared radiation from the sun. At a depth of 5 cm ca. 90% of the near-infra-red (most of the infrared from the sun, and the part with the greatest energy) is adsorbed, resulting in heating the algae above their optimum growth temperature in many environments, and at 50 cm 99% would be adsorbed resulting in considerable heating. At 5 mm only 9% would be adsorbed in a floating bioreactor, and the rest would penetrate to the water below, easing the cooling and heat exchange.
The bubbling in many photobioreactor (PBR) designs is for two reasons—to mix algae and keep them suspended, and to introduce CO2. The CO2 in previous systems must often be diluted with air because at higher concentrations CO2, the bubbling rate required for mixing would overly acidify the medium. This is especially a problem with larger celled algae, as they settle more quickly than small-celled algae, and more mixing energy is required. Large volumes of CO2-enriched air are thus pumped at high energy costs, losing much of the CO2. The present invention precludes the need for using bubbles for mixing and reduces the cost of CO2, sterile medium, harvesting, and produces less effluent if medium after harvest is not recycled. Methods other than sparge bubbling have also been proposed; e.g. mixing the carbon dioxide with the medium being introduced by co-flowing over a solid substrate, and (un-economically) adding NaOH to the medium to capture atmospheric CO2 and thereby generate bicarbonate (EP 2 371 940).
Many closed vertical systems constructed above ground are made of rigid or flexible sheets, tubes, plastic bags/sleeves, or glass walls are described in Table 1. Such structures allow more concentrated growth, and use efficient (but high compression cost) bubbling of carbon dioxide mixed air. The capital costs of the rigid materials are high, as are superstructure costs to assure that they will not be destroyed in high winds. Evaporative cooling from the culture media is impossible in closed systems, and as the water in the structures absorbs infrared light, and cooling can be expensive. Short optical paths can be designed in such systems, allowing increased density of algae (Table 1). Horizontal or near horizontal systems (Table 1) allow for less superstructure. One system (US2007/0048848) uses recumbent flexible plastic sleeves with mixing affected by a track support of peristaltic rollers, with no explanation of how temperature is to be controlled. In another (dimensionless) system, a gas plug is moved through channels by somehow tilting the system to move a gas plug along through the system as a standing wave (US2011/0281339). The density of algal cells and method of cooling is not disclosed therein, and there are superstructures required to perform the tilting.
Totally horizontal systems (Table 1) using plastic film are far less expensive, and are used floating on the sea, where wave motion provides some mixing and the seawater provides the cooling. Both are appropriate only for fresh water algae as they achieve their buoyancy by floating the bioreactors on seawater, using the specific gravity differences to keep them afloat. Carbon dioxide mixed with air is pressure bubbled through the system using spargers, and significant amounts are wasted, as in the other systems. An optical path of 10-15 cm is needed to optimally use the carbon dioxide. There is no horizontal system reported where the depth of algae is less than 5 cm or where carbon dioxide is provided other than by sparging, and where excess oxygen is removed by any process other than venting (Table 1).